Corundum
A hard, crystalline mineral forming rubies and sapphires.
Corundum is a crystalline form of aluminium oxide (Al2O3) that typically contains traces of iron, titanium, vanadium, and chromium. It is a rock-forming mineral and a naturally transparent material, but can exhibit different colors depending on the presence of transition metal impurities. Corundum has two primary gem varieties: ruby and sapphire. Rubies are red due to chromium, while sapphires display a range of colors based on the transition metal present; a rare pink-orange variety is called padparadscha sapphire. The name 'corundum' derives from the Tamil-Dravidian word kurundam (ruby-sapphire).
- Mohs hardness
- 9.0
- Density
- 4.02 g/cm³
- Crystal system
- Trigonal
- Space group
- R3c
Lore & Background
Corundum occurs in metamorphic terranes such as mica schist, gneiss, and some marbles, as well as in low-silica igneous syenite and nepheline syenite intrusives. It is also found adjacent to ultramafic intrusives, associated with lamprophyre dikes, and as large crystals in pegmatites. Due to its hardness and resistance to weathering, it commonly appears as a detrital mineral in stream and beach sands. Corundum for abrasives is mined in Zimbabwe, Pakistan, Afghanistan, Russia, Sri Lanka, and India; historically it was mined from deposits in North Carolina, US, and from a nepheline syenite in Craigmont, Ontario. Emery-grade corundum is found on the Greek island of Naxos and near Peekskill, New York.
Reader's Guide
Corundum's significance stems from its exceptional hardness (9.0 on the Mohs scale), which makes it useful as an abrasive in sandpaper and machining tools. Emery, a black granular variety mixed with magnetite, hematite, or hercynite, is commonly used for this purpose. Synthetic corundum has a lower environmental impact than natural corundum by avoiding destructive mining, but its production is energy-intensive and involves chemicals that can pose risks. Beyond ornamental uses, synthetic corundum is used for mechanical parts, scratch-resistant optics, watch crystals, satellite instrument windows, laser components, and ceramic armor. Its high density (4.02 g/cm³) is unusual for a transparent mineral composed of low-atomic mass elements. The corundum structure type is also found in various binary and ternary compounds.
Did You Know?
- Synthetic corundum can be grown by flame fusion, flux-growth, and hydrothermal synthesis.
- The KAGRA gravitational wave detector uses main mirrors made of 23 kg sapphires.
Frequently Asked Questions
Who is Corundum?
Corundum is a naturally occurring crystalline mineral made up of aluminium oxide (Al₂O₃), commonly found in igneous and metamorphic rock. It is the parent mineral behind two of the world's most celebrated gemstones: ruby and sapphire.
What colors can Corundum appear in?
Depending on which trace transition metals are locked into its lattice, corundum shows up as red (ruby, from chromium), blue, yellow, green, purple, or colorless (sapphire). A rare pink-orange variety is specifically called padparadscha sapphire.
Where does the name 'Corundum' come from?
The term traces back to the Tamil-Dravidian word 'kurundam,' a South Asian name for ruby and sapphire. It entered European mineralogical vocabulary through centuries of gem trade linking the Indian subcontinent to the Mediterranean.
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